1. Field of the Invention
The present invention generally relates to a television telephone apparatus (so-called "TV phone") capable of transmitting/receiving an image via a telephone line. More specifically, the present invention is directed to a TV phone capable of transmitting voice data and image data related to a color intermittent still image.
2. Description of the Related Art
According to the recent development of highly information-oriented society, there is an increasing demand for communication media by which a vast amount of various types of information are transmitted quickly. To fulfill such a demand in Japan, the "analog telephone band still picture video communication system" was established in 1988 as a standard communication system to allow for communication of a monochromatic image between parties over a communication line. The standard was amended in 1989 to enable a color image as well as a monochromatic image can be exchanged. Various television telephone apparatuses (TV phones) which conform to those standards have been developed so far. The TV telephone apparatuses are expected to become popular because they enable visual information to be sent to cover voice-based information or explain what would otherwise be difficult to understand or visualize and because telephone communication is made possible while watching the caller's or receiver's expressions.
TV telephone apparatuses are classified by types of communication networks to be linked and the types of image and voice transmission functions. For example, some TV telephone apparatuses are connected to an analog public telephone line to transmit a monochromatic still picture or a color still picture, or to transmit a color dynamic image. The still-picture TV telephone apparatus, which is connected to an analog public telephone line to send a monochromatic still picture, employs a system for directly transmitting image data without compression. For monochromatic image data of 64 gray scales (6 bits) with one screen consisting of 100.times.160 pixels, for example, data of about 100.times.160.times.6=96000 bits (12 Kbytes) has been transmitted at a transfer rate of about 8740 bps (bit per second). This takes about 11 seconds to send one screen of image data.
Image data carries a large amount of information so that direct processing of image data is not practical in view of memory capacity, communication speed and so forth. As a solution, there has been proposed a color dynamic-image TV telephone apparatus which compresses image data and voice data to about 1/20 to 1/100 before transmission to thereby ensure transmission and reception of a dynamic image at a rate of two to ten frames per second using an analog telephone band.
As the compressed transmission of image data will transfer a considerable amount of data, a slight transmission delay will not affect the progression of telephone communication so much. Therefore, variable length coding (e.g., variable length coding system which conforms to H. 261 of the CCITT (Comite Consultatif International Telegraphique et Telephonique) regulations) is typically employed.
In compressed transmission of voice data, digitalization of voice data can compress the length of the average produced bits. Generally speaking, if entropy compression (e.g., Hoffman coding) which does not guarantee the maximum bit length is adapted for voice signals, a large transmission delay of voice signals occurs when voice signals are converted to a long bit length by the entropy compression. This would greatly affect the progression of telephone communication. In this respect, voice data is encoded with a fixed bit length before transmission.
TV telephone apparatuses which are connected to an analog communication network have a communication function to transmit/receive image data and voice data in multiplexed form. A communication protocol corresponding to the communication networks to be linked and a coding system for image data and voice data, which are affixed to a protocol-based communication signal for transmission/reception, are specified for each type of communication network by the CCITT regulations, etc.
The conventional TV telephone apparatuses generally are arranged with telephones, cameras, and display units in an integral form, as described in U.S. Pat. No. 5,077,784. Accordingly, since the size of the display unit should be made small, when a plurality of images are simultaneously displayed on the plural subdivided screens, respectively, these displayed images are not easily visible.
On the other hand, as disclosed in U.S. Pat. No. 4,985,911, the television telephone apparatus is connectable to the presently existing telephone is the detachable manner. In this type of conventional television telephone apparatus, since the display area of the display unit would be small, when a plurality of images are simultaneously displayed on the relevant subdivided display areas, these displayed images cannot be easily observed.